The present invention relates to improvements over an emergency stop apparatus for electric elevators controlled by the Static-Leonard system.
By convention, the Ward-Leonard system in which a DC motor is directly driven by a motor-generator has generally been used for the speed control of the elevator when a DC motor is used as the motor for driving the cage of the elevator. With recent development of semiconductor devices, however, the called Static-Leonard system employing static components instead of the dynamic apparatus of the motor-generator is put into practice.
The Static-Leonard system suffers from some problems relating to a momentary power stoppage, i.e. a very brief stoppage of power, when compared to the Ward-Leonard system. In the case of the Ward-Leonard system, when the AC power source is momentarily stopped in its power supply, the rotating energy of the motor-generator impedes a rapid reduction of the output voltage thereof. Therefore, the rotation of the motor continues and the cage sustains its traveling. Thus, use of the Ward-Leonard system is accompanied with little problem. On the other hand, in the Static-Leonard system, the power stoppage of the AC power source makes it impossible to control the thyristors of the static components. Further, a DC voltage is developed in the armature of the DC cage drive motor immediately after the power stoppage, and this causes a large short-circuiting current to flow through the thyristor and the AC power source. This excessive current drive a safety circuit including the excessive current relay and necessary components and the safety circuit driven brakes the motor to cause an emergency stop of the travel of the cage. Generally, when the safety circuit having, for example, the excessive current relay is operated, it is dangerous to move the cage. For this reason, the system is so designed that it is impossible to automatically restart the cage travel. Thus, when the safety circuit of this kind operates, it is impossible to restart the travel of the cage by operation from the cage inside. Accordingly, the passengers are confined to the cage. And the passengers are placed in uneasiness for a long time.